This invention relates in general to the recovery of energy from below the Earth's surface and more particularly to improvements in such energy recovery systems.
Within the past few decades commercial use of geothermal energy has acquired a greater status and interest because of the long range dwindling of fossil fuel sources of energy and tragic experiences with nuclear power plants. Current technology associated with geothermal energy conversion has primarily been directed to the extraction of heat from readily available natural heat sources located beneath the Earth's crust and readily exposed by the drilling of relatively shallow wells. These wells are drilled to the depth of the Earth's isothermal zone, a geothermal heat source, which is essentially comprised of a mixture of hot liquid and vapor brine. Geothermal reservoirs have moderate (250.degree. F. to 350.degree. F.) to high (greater than 350.degree. F.) temperatures at the isothermal zone. Geothermal energy recovery systems commonly use fluids, such as water, to extract heat from the brine. This is accomplished by pumping a fluid through pipes contained in a well casing. The fluid absorbs heat from the isothermal zone, and energy is recovered as the fluid, or media heated by the fluid, flows through a workload, such as a turbine.
A variety of known methods exist for conversion of geothermal energy. One method is to force hot vapor and brine to the Earth's surface by the drilling of pipes into the ground that connect to geothermal heat sources. The pressure of these heat sources can force hot vapor and even liquid brine to flow up such pipes to ground level. Heat exchangers are used at ground level to extract heat from such vapor and brine.
But this method is problematic. Vapor derived from such wells is wet steam, a saturated mixture of steam and liquid. Such natural steam carries suspended elements which are frequently not compatible with the moving and stationary components of a turbine. Natural geothermal steam typically needs additional processing before it can be used to drive or supply heat to machinery. In brine producing wells, the liquid brine also suffers considerable heat loss before it reaches ground level. Spent brine is generally returned underground to absorb more heat.
Another technique involves placing heat exchangers in downhole submerged positions. One problem with this method is the task of regulating the movement of heat transfer fluids in order to achieve optimum energy output. When too much heat transfer fluid passes through the heat exchanger, the product vapor is a saturated mixture of steam and liquid, which is frequently incompatible with the components of a turbine. Conversely, when too little moves through the heat exchanger, energy output from the system is too low to be commercially feasible. Another problem is that fluid tends lose heat as it exits the well. Thermally insulating the exit pipe is difficult and expensive.
There is thus a need for a geothermal energy recovery system that provides a method and apparatus to facilitate heat exchange at the brine level while at the same time reducing heat loss as heated fluid is brought out of the ground. Such reduced heat loss results in a higher thermal efficiency of the overall power cycle. There is also a need for a method of regulating the flow rate and pressure of heat transfer fluids so that the fluids vaporize within the heat exchanger to produce a more usable product vapor.